Liquid Crystal Optical Device Configured to Reduce Polarization Dependent Loss and Polarization Mode Dispersion

ABSTRACT

An LC-based optical device compensates for differences in optical path lengths of polarization components of input beam. As a result, PDL and PMD of the optical device are reduced. The compensation mechanism may be a glass plate that is disposed in an optical path of a polarization component so that the optical path length of that polarization component can be made substantially equal to the optical path length of the other polarization component that traverses through a half-wave plate. Another compensation mechanism is a birefringent displacer that has two sections sandwiching a half-wave plate, wherein the two sections are of different widths and the planar front surface of the birefringent displacer can be positioned to be non-orthogonal with respect to the incident input light beam.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. application Ser. No.12/475,116, filed May 29, 2009, which is hereby incorporated byreference herein and made a part of this specification.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present invention relate generally to opticalcommunication systems and components and, more particularly, to a liquidcrystal-based optical device that is configured to reduce polarizationdependent loss and polarization mode dispersion.

2. Description of the Related Art

In optical communication systems, it is sometimes necessary to perform1×2 switching of an optical signal, where an input light beam enters anoptical switching device through an input port and is directed to one oftwo output ports. There are also more complicated optical switchingschemes, such as 2×2, 1×N, and N×N optical switches, which may berealized by combining multiple 1×2 optical switches.

In addition to routing of signals by optical switches, attenuation ofsignals in optical communication systems is needed, for example in anoptical communication system that employs wavelength divisionmultiplexing (WDM). In such an optical system, information is carried bymultiple channels, each channel having a unique wavelength. WDM allowstransmission of data from different sources over the same fiber opticlink simultaneously, since each data source is assigned a dedicatedchannel. The result is an optical communication link with an aggregatebandwidth that increases with the number of wavelengths, or channels,incorporated into the WDM signal. In this way, WDM technology maximizesthe use of an available fiber optic infrastructure, such that what wouldnormally require multiple optic links or fibers instead requires onlyone. In practice, different wavelength channels of a WDM signaltypically undergo asymmetrical losses as they travel through an opticalcommunication system, resulting in unequal intensities for each channel.Because these unequal intensities can compromise the integrity of theinformation carried by the WDM signal, an optical device or array ofoptical devices is used in WDM systems to perform attenuation toequalize the respective intensities of the channels contained in a WDMsignal.

Liquid crystal (LC) based optical switches are known in the art forswitching and attenuation of the channels contained in a WDM signal. AnLC-based optical switch relies on rotating the polarization state oflinearly polarized input beam to perform switching and attenuationfunctions. The LC-based optical switch divides an input beam into s- andp-polarized components, and manages the switching and attenuation ofeach component separately. The division of the input beam into s- andp-polarized components produces two negative effects that need to becompensated. The first is polarization dependent loss (PDL). The s- andp-polarized components experience different losses as they pass throughvarious elements of the LC-based optical switch. The second ispolarization mode dispersion (PMD). PMD occurs because of randomimperfections and asymmetries in the optical medium that is traversed bythe s- and p-polarized components. For optimal performance of theLC-based optical switch, the PDL and the PMD need to be minimized.

SUMMARY OF THE INVENTION

One of keys in reducing PDL and PMD in an LC-based optical switch is tominimize the differences in the optical paths traversed by the s- andp-polarized components. One or more embodiments of the present inventionprovide an LC-based optical device that is configured to reduce PDL andPMD and a method for reducing PDL and PMD in an LC-based optical device.

An optical device according to an embodiment of the present inventionincludes a birefringent displacer for dividing an input beam into afirst component and a second component, an LC structure for conditioningthe polarization state of incident light and disposed in optical pathsof the first and second components, a half-wave plate disposed in theoptical path of the first component between the birefringent displacerand the LC structure, and a glass plate disposed in the optical path ofthe second component between the birefringent displacer and the LCstructure.

An optical device according to another embodiment of the presentinvention includes a birefringent displacer for dividing an input beaminto a first component and a second component, an LC structure forconditioning the polarization state of incident light and disposed inoptical paths of the first and second components, and a half-wave platedisposed in the optical path of the first component between thebirefringent displacer and the LC structure, wherein the birefringentdisplacer includes a first birefringent crystal and a secondbirefringent crystal and the first and second birefringent crystals havedifferent thicknesses.

A wavelength selective switch according to an embodiment of the presentinvention includes a wavelength dispersive element for separating aninput beam into its wavelength components, a birefringent displacerdisposed in optical paths of the wavelength components, an LC structurefor conditioning the polarization state of incident light and disposedin optical paths of the wavelength components, and a half-wave platedisposed between the birefringent displacer and the LC structure,wherein the birefringent displacer includes a first birefringent crystaland a second birefringent crystal and the first and second birefringentcrystals have different thicknesses.

A method for compensating for PDL in an LC-based optical device having abirefringent displacer includes the steps of measuring the PDL of theinput beam, rotating the birefringent displacer, and measuring the PDLof the input beam after rotation of the birefringent displacer. Thesteps of rotating and measuring may be carried out until the measuredPDL is at a minimum. After the minimum PDL is found, the birefringentdisplacer is affixed to its mounting frame so that the angle formed byits front planar surface with respect to an optical path of the inputbeam can be maintained during operation.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1A is a schematic side view of a wavelength selective switch (WSS)that performs 1×2 switching and attenuation of a WDM signal, accordingto an embodiment of the invention

FIG. 1B is a schematic top view of a wavelength selective switch thatperforms 1×2 switching and attenuation of a WDM signal, according to anembodiment of the invention.

FIG. 2 is a schematic cross-sectional side view of one embodiment of anLC-based switching system that can be used in the WSS of FIG. 1.

FIG. 3 is a schematic cross-sectional side view of another embodiment ofan LC-based switching system that can be used in the WSS of FIG. 1.

FIG. 4 illustrates optical path lengths through a portion of theLC-based switching system of FIG. 2 with a glass plate.

FIG. 5 illustrates optical path lengths through a portion of theLC-based switching system of FIG. 3 without a glass plate.

FIG. 6 is a flow diagram that illustrates the method for PDLcompensation in an LC-based optical device.

For clarity, identical reference numbers have been used, whereapplicable, to designate identical elements that are common betweenfigures. It is contemplated that features of one embodiment may beincorporated in other embodiments without further recitation.

DETAILED DESCRIPTION

One or more embodiments of the present invention provide an LC-basedoptical device that is configured to reduce PDL and PMD and a method forreducing PDL and PMD in an LC-based optical device. The LC-based opticaldevice may be a wavelength selective switch (WSS) and the presentinvention will be described below in the context of a WSS. However, thepresent invention is applicable to other types of LC-based opticaldevices, such as a reconfigurable optical add-drop multiplexer (ROADM).

FIG. 1A is a schematic side view of a WSS 100 that performs 1×2switching and attenuation of a WDM signal, according to an embodiment ofthe invention. FIG. 1B is a schematic top view of WSS 100. WSS 100 canselectively direct each of the wavelength channels of an input lightbeam to one of two output optical paths. For example, an input lightbeam containing a plurality of wavelength channels enters through aninput fiber and each of the individual wavelength channels may bedirected to one of two output fibers. The terms “top view” and “sideview” and references to the horizontal and vertical directions are forpurposes of description only. One of skill in the art will recognizethat WSS 100 may be configured in any orientation and perform 1×2switching and attenuation as described herein.

As depicted in FIGS. 1A and 1B, WSS 100 includes an optical input port101, optical output ports 102 and 103, beam shaping optics (as furtherdetailed below), a diffraction grating 110, a quarter-wave plate 108 andan LC-based switching system 120. WSS 100 may also include additionaloptics, such as mirrors, focusing lenses, and other steering optics,which have been omitted from FIGS. 1A and 1B for clarity. The beamshaping optics include X-cylindrical lenses 104, 105 and Y-cylindricallenses 106, 107. The components of WSS 100 are mounted on a planarsurface 190 that is herein defined as the horizontal plane for purposesof this description. In the example described herein, planar surface 190is substantially parallel to the plane traveled by light beamsinteracting with WSS 100. Also for purposes of description, theconfiguration of WSS 100 described herein performs wavelength separationof a WDM signal in the horizontal plane (i.e., as illustrated in FIG.1B) and switching selection, i.e., channel routing, in the verticalplane (as illustrated in FIG. 1A).

Optical input port 101 optically directs a WDM optical input beam 171 tothe WSS 100. Optical input beam 171 includes a plurality of multiplexedwavelength channels and has an arbitrary combination of s- andp-polarization. X-cylindrical lens 104 vertically extends inbound beam171, and Y-cylindrical lens 106 horizontally extends inbound beam 171.Together, X-cylindrical lens 104 and Y-cylindrical lens 106 shapeoptical input beam 171 so that the beam is elliptical in cross-sectionwhen incident on diffraction grating 110, wherein the major axis of theellipse is parallel with the horizontal plane. In addition,X-cylindrical lens 104 and Y-cylindrical lens 106 focus optical inputsignal 171 on diffraction grating 110.

Diffraction grating 110 is a vertically aligned diffraction gratingconfigured to spatially separate, or demultiplex, each wavelengthchannel of optical input beam 171 by directing each wavelength along aunique optical path. In so doing, diffraction grating 110 forms aplurality of inbound wavelength beams, wherein the number of inboundwavelength beams corresponds to the number of optical wavelengthchannels contained in optical input beam 171. As shown in FIG. 1B,diffraction grating 110 is depicted separating optical input beam 171into three input wavelength beams λ₁, λ₂, and λ₃. In practice, thenumber of wavelength channels contained in optical input beam 171 may beup to 50 or more. Because the separation of wavelength channels bydiffraction grating 110 takes place horizontally in the configurationshown in FIG. 1B, spectral resolution is enhanced by widening inboundbeam 171 in the horizontal plane, as performed by Y-cylindrical lens106. Upon return of the wavelength beams from LC-based switching system120, diffraction grating 110 also performs wavelength channelcombination, referred to as multiplexing, as depicted in FIG. 1B.Together, X-cylindrical lens 105 and Y-cylindrical lens 107 columnateoptical input signal 171 so that wavelength beams are normally incidentupon entry into LC-based switching system 120. In addition,X-cylindrical lens 105 and Y-cylindrical lens 107 focus returningwavelength beams λ₁ and λ₃ on diffraction grating 110 after the beamsexit LC-based switching system 120.

WSS 100 performs optical routing of a given wavelength channel byconditioning (via LC polarization) and vertically displacing the s- andp-components of a wavelength beam within LC-based switching system 120.Thus, output beam 172 exiting LC-based switching system 120, which isvertically displaced below input beam 171 into LC-based switching system120, includes the wavelength channels selected for output port 102.Similarly, output beam 173 exiting LC-based switching system 120, whichis vertically displaced above input beam 171 into LC-based switchingsystem 120, includes the wavelength channels selected for output port103. Attenuation may also be performed on each wavelength channelindependently for input beam 171 in LC-based switching system 120, asdiscussed further below.

FIG. 2 is a schematic cross-sectional side view of one embodiment of anLC-based switching system that can be used in the WSS of FIG. 1.LC-based switching system 120 separates an incoming wavelength beam intos- and p-polarized components and provides the capability to control thesettings of an LC beam-polarizing structure in order to switch andattenuate the s- and p-components as desired. As illustrated in FIG. 2,p-polarized light is denoted by a vertical bar, and s-polarized light bya dot.

LC-based switching system 120 includes a birefringent displacer 201, aglass plate 203, a half-wave plate 204, an LC beam-polarizing structure202 that has an array of LC pixels (not shown), and a polarizationseparating and rotating assembly 220. Glass plate 203 is positionedabove half-wave plate 204. Birefringent displacer 201 separates incomingwavelength beams into s- and p-polarized components before thecomponents are conditioned by LC beam-polarizing structure 202, andcombines the separate s- and p-polarized components of output wavelengthbeams into their respective output wavelength beams. For clarity, theoptical paths of the input beam components are not illustrated afterthey pass through glass plate 203 and half-wave plate 204.

Birefringent displacer 201 comprises a first birefringent crystal 231and a second birefringent crystal 232. First birefringent crystal 231and second birefringent crystal 232 may each be YVO₄ crystal or otherbirefringent material that translationally deflects incident light beamsby different amounts based on orthogonal polarization states. Firstbirefringent crystal 231 is oriented relative to wavelength beam A sothat light of one polarization state (s-polarization, in the embodimentillustrated in FIG. 2) passes through first birefringent crystal 231without significant deflection and light of the opposite polarizationstate (p-polarization, in the embodiment illustrated in FIG. 2) passesthrough birefringent displacer 101 with the deflection shown. Secondbirefringent crystal 232 is oriented with an optical axis so that anopposite deflection scheme is realized for incident light relative tothe deflection scheme of first birefringent crystal 231.

First birefringent crystal 231 and second birefringent crystal 232 havesubstantially the same thickness. This ensures that the optical pathlengths for a wavelength beam's s- and p-polarized components aresubstantially equal as they pass through birefringent crystal 201. Inaddition, the thickness of glass plate 203 is chosen to ensure that theoptical path length of a wavelength beam's component that passes throughglass plate 203 is substantially equal to the optical path length of awavelength beam's component that passes through half-wave plate 204.This can be done by making the thickness of glass plate 203 to beNw*d/Ng, where Nw and Ng are refractive indices for half-wave plate 204and glass plate 203, respectively, and d is the width of half-wave plate204.

LC beam-polarizing structure 202 includes an array of LC subpixels thatare formed between a plurality of row electrodes and a plurality ofcolumn electrodes. LC subpixels contain an LC material, such as twistednematic (TN) mode material, electrically controlled birefringence (ECB)mode material, etc. The electrodes apply a potential difference acrosseach of LC subpixels, and each LC subpixel conditions polarity ofincident light based on this potential difference. The electrodes aretransparent and may be patterned from indium-tin oxide (ITO) layers.

Polarization separating and rotating assembly 220 includes abirefringent element 221, a quarter-wave plate 222, and a mirror 223. Inthe embodiment of FIG. 2, birefringent element 221 is oriented with anoptical axis so that its deflection scheme for incident light is similarto that of first birefringent crystal 231 (i.e., opposite to thedeflection scheme of second birefringent crystal 232). That is,p-polarized components pass through birefringent displacer 221 with anupward deflection and s-polarized components pass through birefringentdisplacer 221 without significant deflection. Quarter-wave plate 222 ismounted on mirror 223, where mirror 223 reflects incident light, andquarter-wave plate 222 rotates the polarization of incident light atotal of 90° when incident light passes through quarter-wave plate 222twice. Alternatively, in lieu of mirror 223, other optical apparatus canbe devised by one of skill in the art to redirect light that has passedthrough LC beam-polarizing structure 202 and quarter-wave plate 222 backtoward LC beam-polarizing structure 202 and quarter-wave plate 222 for asecond pass.

FIG. 3 is a schematic cross-sectional side view of another embodiment ofan LC-based switching system that can be used in the WSS of FIG. 1.LC-based switching system 320 can be used in place of LC-based switchingsystem 120. LC-based switching system 320 is identical to LC-basedswitching system 120 except for birefringent displacer 301 and glassplate 203 may or may not be provided. Birefringent displacer 301comprises first birefringent crystal 331 and second birefringent crystal332 of differing widths. The width of first birefringent crystal 331,W1, is greater than the width of second birefringent crystal 332, W2. Inone embodiment, the thicknesses differ by (Nw-1)*d/(AN), where Nw is therefractive index for half-wave plate 204; d is the width of half-waveplate 204; and AN is the birefringence of the crystal. In addition, thebirefringent displacer 301 is rotated about the x axis with respect tothe input beam. As a result of such rotation of birefringent displacer301, the optical path lengths of beam component 233 and beam component234 can be made substantially equal. Typically, the amount of rotationis 0 degrees to 9 degrees, so that the front planar face of birefringentdisplacer 301 forms an angle of 0 degrees to 9 degrees with respect toan imaginary plane that is orthogonal to the optical path of the inputbeam.

FIG. 4 illustrates optical path lengths of beam component 233 and beamcomponent 234 in LC-based switching system 120. In this system, sincethe thicknesses of first birefringent crystal 231 and secondbirefringent crystal 232 are the same, any rotation of birefringentdisplacer 201 about the x-axis would not introduce any optical pathlength change for beam component 233 and beam component 234. As aresult, the thickness of glass plate 203 must be precisely controlled sothat there is minimal optical path length difference between the twobeam components 233, 234. In addition, with this configuration, it isnot possible to compensate for any optical path length difference causedby other components such as LC beam-polarizing structure 202.

FIG. 5 illustrates optical path lengths of beam component 233 and beamcomponent 234 in LC-based switching system 320 that does not employglass plate 203. It can be seen that the optical path length of beamcomponent 234 is longer than the optical path length of beam component233 through birefringent displacer 301, because W1 is greater than W2.However, when the optical path length of beam component 233 throughhalf-wave plate 204 is added, the optical path length of beam component234 becomes equal to the optical path length of beam component 233. Inpractice, it is difficult to perfectly control the thicknesses of firstbirefringent crystal 331 and second birefringent crystal 332 because oftolerance in manufacturing. As a result, it is difficult to make theoptical path lengths of beam component 233 and beam component 234 to bethe same. In addition, other components such as LC beam-polarizingstructure 202, may affect the optical path lengths of beam component 233and beam component 234 in different ways. To make the optical pathlengths of beam component 233 and beam component 234 to be equal afterfirst birefringent crystal 331 and second birefringent crystal 332 havebeen manufactured and birefringent displacer 301 has been assembled,birefringent displacer 301 is rotated about the x-axis by θ. The angleof rotation, θ, is adjusted manually until the optical path length ofbeam component 233 through birefringent displacer 301 and half-waveplate 204 and the optical path length of beam component 234 throughbirefringent displacer 301 become equal. After manual adjustment, anadhesive material, such as an epoxy, is applied to affix birefringentdisplacer 301 to a mounting frame so that the angle of rotation, θ, ismaintained during operation of the WSS or any other optical deviceemploying LC-based switching system 320.

FIG. 6 is a flow diagram that illustrates the method for compensatingfor PDL in an LC-based optical device having a birefringent displacersuch as birefringent displacer 301 of FIG. 3. This method is describedin the context of PDL compensation but can be adapted to PMDcompensation by measuring PMD instead of PDL. In step 610, the PDL ofthe optical device is first measured. Then, the birefringent displaceris rotated about the x-axis in step 612. The PDL of the optical deviceis again measured in step 614. Based on the measurement in step 614, itis determined in step 616 if the PDL is reduced or at a minimum, orneither. If the PDL has been reduced, the method returns to step 612. Ifthe PDL has not been reduced or is not at a minimum, the birefringentdisplacer is rotated about the x-axis in the opposite direction (step618) and the method returns to step 614. If the PDL has reached aminimum, the birefringent displacer is affixed to its mounting frame(step 620) and the method ends.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

1. An optical device comprising: a birefringent displacer for dividingan input beam into a first component and a second component; a liquidcrystal (LC) structure for conditioning the polarization state ofincident light and disposed in optical paths of the first and secondcomponents; a half-wave plate disposed in the optical path of the firstcomponent between the birefringent displacer and the LC structure; and aglass plate disposed in the optical path of the second component betweenthe birefringent displacer and the LC structure.
 2. The optical deviceaccording to claim 1, wherein the birefringent displacer includes afirst birefringent crystal and a second birefringent crystal, the firstand second birefringent crystals having substantially the samethickness.
 3. The optical device according to claim 2, wherein thehalf-wave plate and the glass plate have substantially the same opticalpath length.
 4. The optical device according to claim 1, wherein thebirefringent displacer further comprises a half-wave plate disposedbetween the first birefringent crystal and the second birefringentcrystal.
 5. The optical device according to claim 1, wherein thebirefringent displacer has a front face that is planar but notorthogonal with respect to the optical path of the input beam.
 6. Theoptical device according to claim 5, wherein the birefringent displacerincludes a first birefringent crystal and a second birefringent crystal,the first and second birefringent crystals having different thicknesses.7. An optical device comprising: a birefringent displacer for dividingan input beam into a first component and a second component, thebirefringent displacer including a first birefringent crystal and asecond birefringent crystal, the first and second birefringent crystalshaving different thicknesses; a liquid crystal structure (LC) forconditioning the polarization state of incident light and disposed inoptical paths of the first and second components; and a half-wave platedisposed in the optical path of the first component between thebirefringent displacer and the LC structure.
 8. The optical deviceaccording to claim 7, wherein the birefringent displacer has a frontface that is planar but not orthogonal with respect to the optical pathof the input beam.
 9. The optical device according to claim 7, whereinthe first and second birefringent crystals have thicknesses that differby (Nw-1)*d/AN, where Nw and d are the refractive index and thickness ofhalf-wave plate, respectively. AN is the birefringence of the first andsecond birefringent crystals.
 10. The optical device according to claim9, wherein the birefringent displacer has a front face that is planarand forms an angle greater than 0 degrees and less than 9 degrees withrespect to an imaginary plane that is orthogonal to the optical path ofthe input beam.
 11. A method for reducing the polarization dependentloss (PDL) of an input beam that passes through a liquid crystal opticaldevice having a birefringent displacer, said method comprising:measuring the PDL of the input beam; rotating the birefringentdisplacer; and measuring the PDL of the input beam after rotation of thebirefringent displacer.
 12. The method according to claim 11, whereinthe steps of rotating and measuring are carried out until the measuredPDL is at a minimum.
 13. The method according to claim 12, furthercomprising affixing the birefringent displacer on a mounting frame withrespect to an optical path of the input beam after the minimum PDL isfound.
 14. The method according to claim 13, wherein the birefringentdisplacer is rotated so that its front face is not orthogonal withrespect to an optical path of the input beam.
 15. The method accordingto claim 14, wherein the birefringent displacer is rotated no more than9 degrees.